1. Field of the Invention
The present invention relates to frequency extenders for a telephone line. More particularly, but not by way of limitation, the present invention relates to a frequency extender to expand the bandwidth of a dialup telephone line used to carry remote audio programming.
2. Background of the Invention
Virtually every broadcaster, whether radio or television, has at some point in time, felt the need to carry programming originating from a remote location. In response to this need, a number of solutions have been developed. Unfortunately, every method presently used for remote broadcasting suffers from its own set of disadvantages.
Presently radio frequency devices are the favored method for sending programming from a remote location to a studio or transmitter for broadcast. Devices offered for this purpose are often referred to as a “remote pickup unit” or “RPU.”
Perhaps the favored RPU is a microwave link. Such systems have excellent bandwidth, good signal to noise performance, and usually include bi-directional operation. In most cases the microwave RPU is built into a van, SUV, truck, or the like. Since microwave signals are basically line-of-sight in nature, there is normally an extendible mast on the vehicle to raise the antenna high enough to clear obstacles and increase the range. Even so, microwave links have a limited range. In addition to line of sight operation, microwave systems suffer from a number of other limitations which include: the equipment is expensive, so expensive, in fact, that most small market radio stations would be hard pressed to purchase even a single system; there is setup time in extending the mast and aiming the remote antenna towards the receiving antenna; microwave systems require a dedicated vehicle; overhead power lines can pose a significant risk to the operator while extending the mast; and, like all RF devices, there is a potential for interference and fade.
Perhaps the most pervasive RPU is the UHF or VHF two-way radio. While two way radios are available for a number of bands, by far UHF radios are the most popular, typically operating in the vicinity of 450 MHz. These radios offer moderate bandwidth and cost a mere fraction of the cost of microwave systems. Unfortunately, two-ways are particularly subject to interference, especially in large metropolitan areas where the frequency selected by a radio station for its two-way equipment is likely shared with other businesses. As a result, a remote broadcast may be interrupted by other radio operators. Even if a broadcaster's two-way radio frequency is exclusive, use of such radios has become so pervasive that interference from equipment operating on adjacent channels is common place. Furthermore, while two-way radio transmissions are not limited to line of sight like their microwave counterparts, such radios still suffer from limited range and require a significant investment by a broadcaster.
Remote programming may also be sent to a radio station over the public telephone network. A telephone link has virtually unlimited range, is rarely affected by outside noise sources, and requires only a minimal investment. Unfortunately, if a switched line is used, the bandwidth provided by a telephone connection is marginal at best. The frequency response of a telephone line is generally 300 Hz to 3100 Hz. In comparison, the frequency response of an FM radio broadcast is generally 30 Hz to 15 KHz. Audio sent through a phone line is degraded to the point where even the most untrained ear can distinguish it from other programming. In fact, in competitive radio markets some broadcasters refuse to use dialup phone lines to carry any programming, even for live remotes.
Since bandwidth is the principal disadvantage to using the switched telephone network, a number of techniques are used by radio stations to reduce the problem of limited bandwidth. One solution is to employ a dedicated leased telephone line. Leased lines are directly connected between the source and destination locations. While 10 KHz bandwidth may be available with such lines, the costs are substantially higher than with a conventional phone line, the phone company requires some lead time to install and connect the line, and there is usually a minimum period over which the line must be leased. As a result, a leased line is not practical for most remote broadcasting events.
Another solution to the bandwidth problem is the frequency extender. In its simplest form, a frequency extender shifts the source audio up 250 Hz prior to its transmission over the phone lines. At the receiving end, the frequency of the program audio is shifted back down 250 Hz to its original frequency. The magic of a frequency extender lies in the nature of the frequency range provided by the telephone company on a phone line. As previously mentioned, the typical bandwidth of a phone line is 300 Hz to 3100 Hz, a range of just over three octaves. The frequency shifting technique used by a frequency extender shifts the frequency range to roughly 50 Hz to 2850 Hz, or over five and one-half octaves. At the upper end, where frequency range is sacrificed, 250 Hz is a mere fraction of an octave. At the lower end, the added range from 50 Hz to 300 Hz is well over two octaves. As those familiar with such devices will readily appreciate, as a result of frequency extension, the audio exhibits a fuller, richer sound than audio transmitted without the benefit of such extension. Of course, even with the improved sound, the high end of the audio spectrum is still absent from the program.
To improve high-end performance, multi-line extenders are available. These devices use this same frequency-shifting technique to recover higher portions of the audio spectrum, 2800 Hz at a time. Beyond the obvious problems of requiring the simultaneous use of multiple telephone lines, these devices traditionally have required some setup to compensate for variances in the characteristics of each of the phone lines.
More recently, the broadcast industry has turned to digital codecs. Codecs are available for conventional phone lines, ISDN lines, and even for use over the Internet. In a digital codec, program audio is first digitized, then radically compressed, transmitted in digital form by a modem across the telephone network, received by a modem at the receiving end, decompressed, and finally, converted back to analog form. Such devices can yield amazing improvements in the apparent bandwidth. Unfortunately, they also have a number of limitations, including: 1) digital codecs are presently very expensive, at least compared to their frequency-shifting counterparts; 2) the actual digital throughput of a particular connection is unpredictable and can vary widely, not only from connection-to-connection between the same two locations, but even during a single session; 3) the reproduced audio is typically reconstructed through a “model” and is not the actual audio produced so that the result may include spurious sounds not in the original audio, sounds may be lost in the conversion process, and downstream processing of the audio can yield unpredictable and unwanted results; 4) the quality of the audio is dependent on the digital throughput; and 5) long gaps in the program audio can occur if the modems lose synchronization and must re-handshake. Despite the popularity of codecs, the state of the art of digital transmission over the switched telephone network is just not quite ready for audio broadcast purposes.
Yet another method for handling a remote broadcast is via a cellular telephone connection. While a cellular-to-cellular connection is possible, normally a cellular telephone is used to call a conventional dialup line at the radio station. Analog cell phones are rapidly becoming a relic. However, at least as long as signal strength is adequate, the problems encountered with a cellular connection are basically the same as those encountered with a conventional telephone line, specifically bandwidth. Like a conventional connection, this problem may be somewhat relieved through the use of frequency extenders. An additional annoyance with analog cell phones is the occasional switching between cell sites which causes a momentary “hole” in the audio signal.
Presently, the cellular network is transitioning to all digital. Like the digital frequency extender mentioned above, digital cell phones rely heavily on compression techniques to maximize the amount of audio information which can be transmitted at a relatively low bit rate. Unfortunately, these compression techniques produce a received signal which is essentially a synthesis of the original signal. As is well known in the art, as the system becomes congested or as signal strength degrades, the recovered audio often becomes unintelligible. Furthermore, downstream processing of audio transmitted over a digital cellular connection may produce unpredictable results. Present frequency compression technique are generally not well suited for use with digital cellular phones.
It should be noted that many digital cell phones provide a data connection and there are devices which make use of such a connection to transmit compressed and digitized audio via the digital port on the cell phone. Presently the data rates provided through such phones is too low for the transmission of audio information, even when heavily processed, especially in light of the fact that with many phones, the digital connection may be shared among several users, i.e. with a CDPD connection.
Finally, it is a common practice in the field to direct talent over a separate communication channel typically know as an “interruptible feedback” line or “IFB.” Particularly in the television industry, a phone connection, or cell phone, is often used for an IFB even when programming is sent via an RF link. Since the talent receives cues over the IFB, it is important that such cues be readily intelligible. Thus there is a need for systems which will improve the quality of off-line audio used for remote cuing.
Thus it is an object of the present invention to provide a system and method for frequency extension which provides suitable bandwidth over a conventional switched telephone connection.
It is a further object of the present invention to transmit the information in an audio form such that consistent results are provided from one connection to the next.
It is still a further object of the present invention to provide a lowcost frequency extender which substantially doubles the bandwidth of a telephone connection.